Field of Invention
The present invention relates to a light diffusion sheet, and a backlight unit for a liquid, crystal display device.
Description of the Related Art
Backlight systems, which illuminate a liquid crystal layer from behind, are in widespread use as a transmissive liquid crystal display device, and the transmissive liquid crystal display device is equipped with an edge-lit (side-lit) backlight unit, an immediate beneath type backlight unit or the like on the back face side of the liquid crystal layer. The edge-lit backlight unit 41 is generally equipped with a light source 42, an optical waveguide plate 43 in the form of a rectangular plate arranged with its edge aligned with the light source 42, and a plurality of optical sheets 44 superposed on the front face side of the optical waveguide plate 43, as shown in FIG. 5 (see Japanese Unexamined Patent Application, Publication No. 2005-107108). LEDs (light emitting diodes) and cold-cathode tubes and the like are used as the light source 42, and the LEDs are currently in widespread use from the viewpoint of the size reduction, energy saving, and the like. In addition, the optical sheet 44 has optical functions such as diffusion and refraction of the transmitted rays of light and the like, and as the optical sheet 44, (1) a light diffusion sheet 45 which is disposed on the front face side of the optical waveguide plate 43 and primarily has a light diffusion function, and (2) a prism sheet 46 which is disposed on the front face side of the light diffusion sheet 45 and has a refraction function toward a normal direction side, and the like are utilized.
With regard to the functions of the backlight unit 41, in brief; first, the rays of light entering the optical waveguide plate 43 from the light source 42 are reflected on reflection dots or a reflection sheet (not shown in the Figure) on the back face of the optical waveguide plate 43 as well as on each lateral face, and are emitted from the front face of the optical waveguide plate 43. The rays of light, emitted from the optical waveguide plate 43 enter a light diffusion sheet 45, and are substantially uniformly diffused and emitted from its front face. The rays of light emitted from the front face of the light diffusion sheet 45 enter the prism sheet 46, are refracted by a plurality of protruding and streaking prism portions formed on the front face toward a normal direction and emitted, and illuminate the entire liquid crystal layer (not shown in the Figure) located further above.